This invention is in the field of flame-retardants. More particularly, it relates to fire-retardant polyolefin compositions.
Brominated organic compounds are commonly used as additives for retarding and slowing the flammability of plastic compounds they are blended with. They may be blended alone or in combination with other brominated or non-brominated flame-retardants in a synergistic manner. Optionally, additional compounds may be added to the blend in order to achieve good flame-retarding results and maintain durability. In general brominated aliphatic compounds are more effective flame-retardants than brominated aromatic compounds since they tend to break down more easily (International Plastics Flammability Handbook, 2nd edition, Jurgen Troitzsch, p. 45).
GB 2,085,898 discloses a self-extinguishing polyolefin composition containing polypropylene, a brominated arene together with Sb2O3 and a free radical initiator. JP 63/027,543 discloses a flame-retardant polyolefin composition comprising a blend of chlorinated polyethylene and polyethylene, together with an organic brominated compound and Sb2O3, a free radical initiator and a metal hydroxide. U.S. Pat. No. 4,430,467 discloses a self-extinguishing propylene polymer where the propylene polymer is blended with 5,6-dibromonorbornane and a free radical initiator. This flame-retardant has low bromine content and it is not melt blendable.
Another flame-retardant widely used in polyolefins is Tetrabromobisphenol A bis (2,3-dibromopropyl ether) (International Plastics Flammability Handbook, 2nd edition, Jurgen Troitzsch, p. 56), but it suffers from heavy blooming and has limited UV stability. By xe2x80x9cbloomingxe2x80x9d it is meant that a separation of the additive from the polymer matrix occurs, which has a negative effect on the surface appearance of the plastic articles. Many of the above mentioned flame-retardants require the additional use of antimony trioxide as a synergist.
Another flame-retardant used in polyolefins is tris(tribromoneopentyl) phosphate, which is also known as tris(3-bromo-2,2(bromomethyl)propyl) phosphate. Some of the advantages of this flame-retardant are: minimal impact upon the properties and the processing of the polymer; easily extrudable with polypropylene; free flowing powder; melt and mix with polypropylene resin to give a uniform product; exceptional heat stability which results in processing stability, storage stability, and performance permanence; it can be used without antimony trioxide, for instance, for the production of fine denier polypropylene fibers (Proceedings of the Flame Retardants ""96 Conference, p. 107).
Only a combination of very high loadings of tris(3-bromo-2,2 (bromomethyl) propyl) phosphate with antimony trioxide give rise to a composition that may pass the UL (Underwriters Laboratory) 94 V-0 test. Such a composition is not commercially competitive.
WO 98/17,718 discloses adding a halogenated flame-retardant having at least one halogen atom attached to an aliphatic carbon atom to tris(tribromoneopentyl) phosphate in order to achieve V-0 in the UL 94 test in a polyolefin at a low loading. U.S. Pat. No. 5,393,812 discloses a composition of polyolefin, a phosphate or phosphonate ester of a halogenated organic compound and a light stabilizer of a certain type.
It is an object of the present invention to provide non-blooming flame-retardant polyolefin formulations having good flame-retardancy, excellent UV and light stability, and excellent thermal stability.
It is yet another object of the present invention to significantly improve the flame-retardancy of tris(tribromoneopentyl) phosphate, which is known to be less efficient than other commercial flame-retardants in similar applications (Proceedings of the Flame Retardants 2000 Conference, p. 82).
It is a further object of the present invention to achieve very short burning time in polypropylene objects.
Still yet another object of the present invention is to provide a polyolefin with a higher standard of flame-retardancy, such as UL 94 V-0, and excellent thermal stability with a low amount of halogenated aliphatic compounds and without the use of halogenated aromatic compounds.
In addition a further object is to provide a flame-retardant polyolefin formulation containing tris(tribromoneopentyl) phosphate and polyolefin that does not include antimony trioxide.
Thus, the flame-retardant polyolefin composition of the present invention comprises:
(a) At least one polyolefin;
(b) tris(tribromoneopentyl) phosphate; and
(c) free radical source
The polyolefin may be a polymer blend comprising at least 20% (w/w) polypropylene. It may either be a homopolymer or a copolymer. The amount of the tris(tribromoneopentyl) phosphate is preferably in the range of about 0.5% to about 20% (w/w) of the entire composition, and the amount of the free radical source is in the range of between about 0.01% to about 4% (w/w) and preferably in the range of between about 0.05% to about 2% (w/w).
The composition may further comprise another fire-retardant compound, which may serve as a synergist. In such a composition, the amount of the organic brominated compound may be reduced, thus leading to a lower amount of bromine in the composition, which makes the composition more economic.
The present invention deals with flame-retardant polyolefin compositions, which, due to their unique additives to the polyolefin and the percentage in the composition, retain most of the physical characteristics of a pure polyolefin polymer while attaining strict fire-retardance standards. The flame-retardancy properties of the olefin composition are achieved by selecting an appropriate brominated compound and a free radical source. The brominated compound is tris(tribromoneopentyl) phosphate which is also known as FR-370 (manufactured by Dead Sea Bromine Group). The compound is a very stable brominated aliphatic compound, which does not undergo chemical reactions common to aliphatic brominated compounds. This stem from the fact that there is no hydrogen atom bound to the carbon atom, which is in the xcex2-position in relation to the bromine, thus avoiding the possible elimination of HBr.
The free radical source in accordance with the present invention is an organic compound which is stable at processing temperatures of about from 150xc2x0 C. to about 250xc2x0 C., and decomposes above these temperatures (at about from 220xc2x0 C. to about 350xc2x0 C.) to give relatively stable free radicals. Examples of free radical initiators are 2,3-dimethyl-2,3-diphenyl-butane and 2,3-dimethyl-2,3-diphenyl-hexane.
The polyolefins useful in this invention (sometimes also referred to as xe2x80x9cpolyolefin resinsxe2x80x9d) may be derived from a variety of monomers especially from propylene, ethylene, butene, isobutylene, pentene, hexene, heptene, octene, 2-methyl propene, 2-methyl butene, 4-methylpentene, 4-methyl hexene, 5-methyl hexene, bicyclo (2,2,1)-2-heptene, butadiene, pentadiene, hexadiene, isoprene, 2,3 dimethyl butadiene, 3,1 methyl pentadiene 1,3,4 vinyl cyclo hexene, vinyl cyclohexene, cyclopentadiene, styrene and methyl styrene. The polyolefins include copolymers produced from any of the foregoing monomers and the like, and further include homopolymer blends, copolymer blends, and homopolymer-copolymer blends. The polyolefins may be in a molding grade, fiber grade, film grade or extrusion grade
The preferred polyolefins are polypropylene and polyethylene, including atactic, syndiotactic and isotactic polypropylene, low density polyethylene, high density polyethylene, linear low density polyethylene, block copolymers of ethylene and propylene, and random copolymers of ethylene and propylene. The polyolefins useful in this invention may be produced using a variety of catalytic processes including metallocene-catalyzed processes. The polymers may have a broad range of melt flow indexes (MFI) but will typically have MFI values in the range 0.5 to 30. The invention finds particular applications in polymers, which are fabricated into finished articles by molding processes. Preferred grades are fiber grades, film grades, molding grades, and extrusion molded grades.
The addition of tris(tribromoneopentyl) phosphate together with the free radical initiator results in a polyolefin composition having a high degree of flame-retardancy.
This flame-retardancy can be further enhanced by the use of other flame-retardant compounds which may serve as synergists such as antimony compounds (e.g. antimony-trioxide, -tetraoxide, -pentaoxide, and sodium antimonate), tin compounds (e.g. tin-oxide and -hydroxide, dibutyl tin maleate), molybdenum compounds (e.g. molybdenum oxide, ammonium molybdate), zirconium compounds (e.g. zirconium-oxide and -hydroxide), boron compounds (e.g. zinc-borate, barium-metaborate), zinc compounds such as zinc stannate, silicon compounds such as silicon oil, fluoro compounds such as polytetrafluoroethylene, and hydroxystannate or any mixtures of two or more of them. Such compounds serve as synergists, which reduce the overall, required amount of flame-retardant compounds in the polyolefin composition.
The composition may further comprise other halogenated or non-halogenated flame-retardant compounds such as but not limited to tetrabromobisphenol A bis (2,3-dibromopropyl ether), brominated expoxy resins and related end capped derivatives, brominated polycarbonate resins and their end capped derivatives, brominated diphenyl ethers, brominated diphenyl ethanes, tetrabromobisphenol A, hexabromocyclododecane and their various thermally stabilised grades, BT-93 (flame retardant produced by Albemarle), poly (pentabromobenzyl acrylate), tris (tribromophenyl) cyanurate, chlorinated paraffins, chlorinated polyethylene, dechlorane, magnesium hydroxide, alumina trihydrate, ammonium polyphosphate, and melamine derivatives (melamine cyanurate and/or pyrophosphate).
The composition may further comprise additional additives which are known in the art such as ultraviolet and light stabilizers, UV screeners, UV absorbers, release agents, lubricants, colorants, plasticizers, fillers, blowing agents, heat stabilizers, antioxidants, reinforcement (e.g. fibers), impact modifiers, processing aids, and other additives. The UV screeners may be for example TiO2 The ultraviolet and light stabilizers may be from the family of hindered amine light stabilizers (HALS), HALS that are alkoxyamine functional hindered amines (NOR-HALS), or UV absorbers such as benzotriazole or benzophenone or a combination of them. Compositions containing tris(bromoneopentyl) phosphate, free radical initiators, and NOR-HALS have especially good UV stability.
The composition may further comprise additional fillers such as talc, calcium carbonate, mica, carbon black, of fiber reinforcement such as glass fibers and carbon fibers. The composition may further contain additives to improve electrical conductivity of the compounds.
Generally, the compositions of the present invention comprise between about 0.5% to about 20% (w/w) of tris(tribromoneopentyl) phosphate and between about 0.01% to about 4% (w/w) free radical initiator together with a polyolefin, preferably between about 0.05% to about 2% (w/w) free radical initiator together with a polyolefin. In cases where a synergistic compound is used, typically an amount from about 0.5% to about 10% (w/w) of a synergist such as antimony trioxide is used. In cases where another fire-retardant is used, typically an amount from about 0.5% to about 60% (w/w) of the fire-retardant is used.
In some instances, it is preferred to use an antimony trioxide-free system. For instance, in fiber applications in order to avoid the clogging of the spinerette during the fiber extrusion.
In many applications such as production of polypropylene fiber and multifilament, it is preferred to add the flame-retardant composition as a masterbatch concentrate in order to obtain a more homogeneous fiber, where the composition is evenly distributed. Thus the use of masterbatch concentrates allows a more stable production of PP fibers.
The masterbatch concentrate contains about 2% to about 90% (w/w) of tris(tribromoneopentyl) phosphate and about 0.03% to about 12% (w/w) of free radical initiator. Preferably, the masterbatch concentrate will contain about 25% to about 80% (w/w) of tris(tribromoneopentyl) phosphate and about 0.1% to about 10% (w/w) of free radical initiator. The masterbatch can also contain about 0.7% to about 30% (w/w) of antimony trioxide.
Due to the very stable and good mechanical properties, the polyolefin compositions of the present invention may be used in many applications. Non-limiting examples of potential use of compositions of the present invention are fibers for a textile structure, carpets, upholstery, injection products such as stadium seats, electrical parts (connectors, disconnectors and sockets), and electrical appliances, extrusion products such as profiles, pipes, sheets for roofing, films and boards for packaging and industry, insulation for cables and electric wires.
Flame-retardancy was measured using the Underwriters Laboratory standard UL 94. In some cases, a UL 94 V-2 standard is achieved, but with a very long burning time, especially in polypropylene block copolymers, which are known to be rather difficult to convert into flame-retarding copolymers (Proceedings of the Flame Retardants 2000 Conference, p. 89). For many applications, it is desirable to have a polypropylene copolymer with a short burning time. Some of the compositions of the present invention achieve this goal.
The resin composition can easily be prepared by pre-mixing prescribed amounts of a thermoplastic resin and compounding additives in a mixing machine, e.g., a Henschel mixer and a tumble mixer. The mixture is then introduced into an extruder, a kneader, a hot roll, a Banbury mixer, etc. in order to melt the resin and evenly distribute the additive throughout the resin.